Printers and copiers have in recent years been transitioning from analog to digital, and while there is strong demand for an excellent latent image reproducibility and high resolution, there is at the same time strong demand for greater energy savings and higher stability.
Lowering the power consumption in the fixing step of a copier or printer is crucial when greater energy savings are considered here.
The implementation of film fixing to achieve additional reductions in the fixation temperature is an effective method for reducing the power consumption. Film fixing readily supports a reduction in power consumption because it provides an excellent thermal conductivity through the use of a film.
An issue associated with reducing the fixation temperature with film fixing is that the releasability between the toner and film during fixing is inadequate and the toner cannot be fixed to the media, e.g., paper, and the occurrence of development in which a portion of the toner is taken off by the film, so-called “cold offset”, has frequently been observed.
There have been attempts at improving cold offset by focusing on the fixing unit; for example, improvements have been pursued based on the film material and based on methods that control the pressure, pressure distribution, and fixation temperature during fixing.
There have, on the other hand, also been toner-oriented attempts to improve cold offset.
Examples in this regard include lowering the melting point of the release agent and/or adding large amounts of release agent and lowering the molecular weight of the binder resin and/or lowering the glass-transition temperature of the binder resin. These methods do tend to improve cold offset, but additional improvements are required. In addition, there is a tendency with these toners for the developing performance to also be diminished, and in particular a substantial reduction in image stability is quite prone to occur during long-term use.
With regard to improving the toner in order to enhance the stability during long-term use, there have been efforts to reduce the changes in durability by, for example, engineering the method of attaching external additives to the toner particle and engineering the type of external additive.
In Patent Document 1, a toner is disclosed for which the toner particles are produced by the emulsion aggregation of a styrene resin, paraffin wax, and so forth; the external addition method is engineered; and the ratio between the saturation water content HL under low-temperature, low-humidity conditions and the saturation water content HH under high-temperature, high-humidity conditions is brought into a prescribed range.
Controlling the water content in this manner did in fact provide a certain improvement in the transferability and image density reproducibility; however, no reference was made to cold offset and this has been inadequate for obtaining the effects of the present invention.
In Patent Document 2, a stabilization of the developing • transfer steps is devised through control of the total coverage ratio of the toner base particle by an external additive, and in fact a certain effect is obtained for a prescribed toner base particle by controlling a calculated theoretical coverage ratio. However, the actual state of attachment of an external additive is quite different from the value calculated under the assumption that the toner is spherical, and the stability during long-term use, which is the problem identified above, does not correlate with this theoretical coverage ratio and improvement has thus been required.